1. Field of the Invention
The present invention relates to a system for controlling the quantity of codes representative of image data, e.g., a method of compressing image data by orthogonal transform coding and controlling the quantity of resulting codes to a target quantity, and an apparatus for practicing the same.
2. Description of the Background Art
One of conventional systems for controlling the quantity of codes representative of image data divides a single scene of image data into a plurality of blocks, executes orthogonal transform with the image data block by block so as to produce transform coefficients, normalizes the transform coefficients by dividing them by an adequate normalization factor, rounds off the fractions of the resulting quotients for quantization, and codes the quantized values to thereby output compressed data. This kind of control system is disclosed in, e.g., Chen "Scene Adaptive Coder", IEEE Trans. on Comm. Vol. COM-32, No. 3, March 1984, pp. 225-232. To limit the compressed data to a desired target quantity of codes, the system taught in the above document monitors an output buffer temporarily storing the compressed data and changes the normalization factor in accordance with the quantity of codes stored in the buffer. For example, Japanese patent laid-open publication No. 104180/1990 teaches a system which counts, among one scene of image data, the codes of a representative block, selects, based on the resulting count, a particular normalization factor confining the compressed data to a desired target quantity of codes, and repeats such a procedure so as to limit the compressed data to a target quantity. However, the problem with the representative block scheme is that the quantity of codes depends on how the representative block is selected.
Japanese patent laid-open publication No. 10048711990 proposes a so-called feedback system which transforms one scene of image data to transform coefficients by orthogonal transform, normalizes and quantizes the transform coefficients by using a certain normalization factor and repeats, if the quantity of codes is not confined to a target value, the above procedure with another normalization factor. This kind of simple feedback scheme, however, has a drawback that feedback must be repeated a number of times until compressed data satisfying the target quantity have been achieved, consuming a substantial period of time, as will be described specifically later.